The present invention relates to the field of superconductors and more particularly, to a method for measuring the critical current of superconductors.
A superconductor is a material that, below some temperature usually referred to as the critical temperature, T.sub.c, loses all electrical resistance to the passage of DC electric current. That is, while superconducting, there is no dissipation of energy; the electrical conduction is lossless. Stated another way, current, once flowing in a loop of such superconducting material, could flow indefinitely without attenuation. A superconductor will only remain superconducting, however, below its critical temperature, below some critical applied magnetic field, and only for electric currents below some critical current. If that critical current is too low, the material cannot be used for most practical applications.
Superconducting materials are useful because their critical currents can be very large over 1,000,000 Amps/cm.sup.2. Superconducting wires can be formed into coils to form magnets that can produce magnetic fields many times that of the largest iron electromagnets, and they can do so in a package small enough to be held in the palm of one's hand.
Superconductors have many applications in electronics. Superconducting quantum interference devices (SQUIDs) may be used to make ultra-sensitive magnetometers and gradiometers, digital logic and memory circuits, and radiation detectors covering many parts of the electromagnetic spectrum. Also, superconducting interconnects on microelectronics packages and chips could be used to reduce losses and dispersion for a diverse mix of microelectronic circuits. The actual current needed for these applications is usually in the range of milliamps rather than thousands of amps. However, microelectronics features are being continually reduced in size, and consequently, the required current densities for microelectronics applications are actually comparable to those required for large scale applications such as motors, generators, and energy storage systems. Thus, if superconducting films cannot carry appreciable electric current densities, they will be useless for any real microelectronics circuit. Thus, the ability to measure the critical current of superconductors is a necessary prerequisite to incorporating these materials into practical devices.
In late 1986, high temperature ceramic superconductors were discovered. These materials are very different than previously known superconductors, such as niobium (Nb), niobium-titanium (Nb-Ti), and niobium germanium (Nb.sub.3 Ge), because they are superconducting at much higher and more easily attained temperatures. Another major difference is that there is more than one measure of the current carrying capacity of the ceramic superconductors--their so-called critical current density, J.sub.c. This was not generally the case for the previously known superconducting materials. This fact was not universally appreciated in the scientific community during the 1987-1988 time period. Reports of critical current measurements differed widely around the world, depending on who reported the results, the samples used, and most notably, the measurement techniques employed. Many different techniques have been developed over the years in order to measure the critical current density of superconducting materials. For the conventionally known materials, it really didn't matter a great deal which measurement technique was used because these materials usually had only one critical current density.
The reason the new ceramic superconductors can be characterized by two different critical current densities has to do with their unique morphology, which results from the way they are synthesized. High quality grains of superconductor, carrying large currents, are separated by lower quality material, referred to as weak links, which carry low levels of current. These intergrain regions may consist of materials that are off-stoichiometry, under-oxygenated or which contain impurities and reaction by-products. Transport techniques used to measure J.sub.c 's by passing a DC electric current through the sample, are thereby limited by the lower critical current, thus defining a weak-link intergrain current density. Non-transport methods, such as magnetic hysteresis, can be used to infer the intrinsic current in the individual grains (intragrain critical current density). Discrepancies in the reported critical currents of ceramic superconducting materials due to the existence of two characteristic J.sub.c 's resulted in much confusion and a lack of progress toward developing practical applications for these materials.
For most applications, the difference between these two measures of the critical current density is an indicator of material defects which limit the performance of the superconductor. Ideally, there should be just one, large critical current for a given material. That is, for a well prepared sample of ceramic superconductor, measurements of the weak-link, intergranular J.sub.c and of the intrinsic, intragranular J.sub.c should yield the same value, indicating that the intergranular regions are not limiting the current-carrying capacity of the sample.
The standard technique for determining the critical current I.sub.c of a superconductor consists of applying a constant or direct current (DC), I, until the voltage difference, V, which appears across the sample exceeds a given value. The current at this point is operationally defined as the critical current. The value of I.sub.c determined from such an experiment can depend on the voltage (or electric field or resistance) criterion chosen. This DC method, although widely used, has the following drawbacks: (1) I.sup.2 R heating of the sample and contacts (with total resistance R) can give a misleadingly low value for I.sub.c, (2) no information is obtained on the rest of the superconducting-to-normal transition, only on the onset of resistance, and (3) special sample mounts (heavy wires, etc.) are needed to carry the large direct currents involved in measurements on bulk samples.
Pulsed current techniques have been used in the past to overcome these problems in measuring the critical current. For the most part, these efforts have used specially designed circuits for producing high-amplitude, multi-step current pulses, or have been limited to single pulses measured from an oscilloscope screen. More recently, in Phys. Rev. B 39, 9139 (1989), Goldschmidt describes a quasi-DC "pulse" technique in which current is switched on for about 0.5 seconds, and then turned off for about 5 seconds to allow for heat dissipation. Microscopically, during the period the current is switched on, the normal (non-superconducting) part of the sample and the current contacts to the sample will heat up and warm the superconducting grains, perhaps even driving them normal. The problem is compounded for high currents since the heat generated in the sample increases as I.sup.2. One problem that his method does not address is that it does not permit a systematic measurement of the entire transition between the normal (resistive) state and the superconducting (lossless) state, particularly at high currents, or of a determination of the intragranular critical current. Furthermore, the maximum current used by Goldschmidt was 1A. His method would lead to an unacceptable level of sample heating at higher currents.
T. E. Jones and W. C. McGinnis disclosed an abstract of a pulsed current technique to derive the critical temperature of a mixed phase sample of Yb-Ba-Cu-O (D. U. Gubser, M. Schuter, Materials Research Society Extended Abstracts, High Temperature Superconductors, Vol. EA-11, "Critical Current Measurements On Yb-Ba-Cu-O", 1987). Pulsed current was used to reduce the effects of sample heating. However, since this method used an AC voltmeter, only the time-averaged value of the sample voltage was measured. The technique presented in that abstract had not been refined to the point where the measurement could be done without noticeably heating the sample. The critical current could not be measured directly, but rather had to be inferred by plotting the suppression of T.sub.c (taken as the resistive transition mid-point) vs. the duty cycle of the current pulses on a graph, where the intercept on the T.sub.c axis (extrapolation to zero duty cycle) was the true T.sub.c of the material, thus accounting for sample heating.
A pulsed transport technique for determining the resistance of a superconductor as a function of temperature is presented in "Critical Current Densities for the High Temperature Ceramic Superconductors YBa.sub.2 Cu.sub.3 O7 and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10+x ", IEEE Transactions on Magnetics, Vol. 25, No. 2, March 1989. The low duty-cycle, pulsed technique was used on specially prepared samples with low resistance current contacts which allowed a determination of the entire superconducting/normal phase boundary without the problems associated with sample heating. However, that method had the shortcoming of having to measure the sample voltage while the temperature was changing. This provided a measurement of the transition temperature of the sample under pulsed conditions, rather than being a direct measurement of the critical current at a fixed temperature.
Thus, there is a need for a simple and repeatable method for directly measuring the intergranular, weak-link critical current and the intrinsic, intragranular critical current of a superconductor.